Heat the oil in a 10-inch skillet over high heat. Add the onions, red bell peppers, and the Anaheim peppers and cook, stirring almost constantly, for 4 minutes. Add the stock and bring to a boil. Add the jalapeños and garlic, and stir well. Remove from the heat and purée in a food processor or blender. Return the purée to the skillet and bring almost to a boil, then whisk in the butter and brown sugar. Remove from the heat.